Lactic acid is widely applied not only to uses such as food and pharmaceuticals, but also to industrial uses as a monomer material for plastics, and has been increasingly demanded. 2-Hydroxypropionic acid, that is, lactic acid, is known to be produced by microbial fermentation, wherein microorganisms convert substrates containing hydrocarbons such as glucose into lactic acid. Lactic acid is divided into optical isomers, the (L)-isomer and the (D)-isomer, based on the conformation of the substituent bound to the carbon at the a-position of carbonyl. By appropriately selecting the microorganism for microbial fermentation, (L)- or (D)-lactic acid can be selectively produced, or lactic acid as a mixture of the (L)-isomer and the (D)-isomer (racemic body) can be produced.
Production of lactic acid by microbial fermentation is generally carried out while a pH appropriate for the microbial fermentation is maintained by addition of an alkaline substance (e.g., calcium hydroxide) to the culture broth. Most of the lactic acid produced as an acidic substance by the microbial fermentation is present as a lactic acid salt (e.g., calcium lactate) in the culture broth due to the addition of the alkaline substance. In general, when lactic acid is used as monomers for a plastic, the lactic acid is preferably in the free form obtained by adding an acidic substance (e.g., sulfuric acid) to the culture broth after completion of the fermentation. However, the lactic acid fermentation broth obtained by microbial fermentation contains, besides lactic acid as the product of interest, organic acids and salts thereof, proteins, amino acids, and nonionic compounds such as glycerol, as impurities. Thus, when the lactic acid is used as monomers for a plastic, the lactic acid needs to be separated from these impurities.
As a method of removing various impurities from a lactic acid-containing solution derived from a lactic acid fermentation broth obtained by microbial fermentation, and recovering lactic acid, a method based on crystallization of lactic acid is known. In that crystallization, lactic acid is precipitated as crystals to increase the chemical purity as well as the optical purity of the lactic acid, and most of the fermentation-derived impurities are distributed into the liquid component (mother liquor). Since, normally, a large amount of lactic acid remains in the mother liquor together with impurities, industrial-scale production of lactic acid from a lactic acid fermentation broth requires enhancement of the purification yield by recycling of the mother liquor to any of the preceding steps. As an example of purification of lactic acid by crystallization, JP 2010-189310 A discloses a method in which a lactic acid-containing solution is diluted and hydrolyzed, followed by performing crystallization to increase the yield. Japanese Translated PCT Patent Application Laid-open No. 2002-540090 discloses a method in which a lactic acid-containing solution is distilled, and crystallization is then performed to recover high-quality lactic acid. However, those methods do not concretely describe a process of recycling the mother liquor after the crystallization.
It could therefore be helpful to provide a method of efficiently and stably producing highly pure lactic acid by recovering lactic acid from a lactic acid-containing solution by crystallization, and suppressing accumulation of multimeric lactic acid and impurities in the recycle system for the mother liquor.